Filter Switching System and Method

ABSTRACT

A communication system with variable filter bandwidth includes a first mixer circuit configured to receive a communication signal and shift the frequency range of the communication signal to a first frequency range. A second mixer circuit is configured to receive the same communication signal and shift the frequency range of the communication signal to a second frequency range. An activation circuit is coupled to the first and second mixer circuit so as to provide an activation signal that activates at least one of the mixer circuits. A plurality of filter circuits are provided such that each filter circuit is configured to receive a signal from a corresponding mixer circuit, when said corresponding mixer circuit is activated.

FIELD OF THE INVENTION

This invention relates to communication systems and more specifically tocommunication transmitters and receivers which are capable of switchingtheir channel frequency bandwidth.

BACKGROUND OF THE INVENTION

In many communication applications it is desirable to employ radioreceivers and transmitters which utilize variable filter bandwidths. Forexample, indoor wireless telephones and wireless local area networksWLANs, require such variable filter bandwidths.

Typically, systems that employ variable filter bandwidths are designedto include a plurality of filters with different frequencycharacteristics. One filter or a group of filters are thenelectronically selected to process a communication signal by employingone or more switches that route the signal to the appropriate set offilters.

As will be explained in more detail in reference with FIG. 1, suchfilter switching arrangement may be implemented, for example, infrequency multiplication stage of a transmitter or a receiver system.Typically, a mixer is employed to shift the frequency range of anincoming signal. In the case of a receiver, the mixer is used to shiftdown a high frequency signal to a lower frequency range. A switch isconfigured to receive the output signal from the mixer and route thisoutput signal to an appropriate filter.

Such filter switching arrangement may also be implemented, for example,in a signal amplification stage. Typically, two or more filters withdifferent frequency characteristics are configured to receive anamplified voltage signal via a switch.

One disadvantage with this switching configuration is that the use ofswitches in combination with mixers or amplifiers introduces signaldistortions that leads to higher error rates. Furthermore, thetermination impedances of each filter require to be substantially thesame and match with the termination impedance of the mixer or amplifieroutput stage. However, it is sometimes difficult to design filters withdifferent frequency bandwidths that exhibit substantially the sametermination impedance.

Thus, there is a need for a communication system that employs variablefilter bandwidths and has substantially no distortion due to the use ofswitches for routing signals to various filters.

SUMMARY OF THE INVENTION

In accordance with an exemplary embodiment of the present invention, acommunication system with variable filter bandwidth comprises a firstmixer circuit configured to receive a communication signal and shift thefrequency range of the communication signal to a first frequency range;a second mixer circuit configured to receive the communication signaland shift the frequency range of the communication signal to a secondfrequency range; an activation signal coupled to the first and secondmixer circuit so as to activate one of the mixer circuits; a pluralityof filter circuits each configured to receive a signal from acorresponding mixer circuit, when the corresponding mixer circuit isactivated.

In accordance with another exemplary embodiment of the presentinvention, a communication system with variable filter bandwidthcomprises a plurality of Gilbert cells configured to receive acommunication signal and a multiplying signal; a plurality of filtershaving a prespecified bandwidth such that each of the filters isconfigured to receive a signal from a corresponding Gilbert cell.Furthermore, each Gilbert cell is configured to go to an active statewhen it receives an activation signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a communication system withvariable filter bandwidth in accordance with one embodiment of thepresent invention;

FIG. 2 illustrates a schematic diagram of a prior art mixer circuittypically known as a Gilbert cell;

FIG. 3 illustrates a schematic diagram of a mixer cell employed in acommunication system with variable filter bandwidth in accordance withone embodiment of the present invention.

FIG. 4 illustrates a schematic diagram of a mixer cell employed in acommunication system with variable filter bandwidth in accordance withanother embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of a communication system 10 with variablefilter bandwidth in accordance with an exemplary embodiment of thepresent invention, although the invention is not limited in scope inthat respect. Communication system 10 may be a receiver that isconfigured to receive frequency signals via antenna 12. The output portof antenna 12 is coupled to an input port of a high frequency mixerstage 16.

High frequency mixer stage 16 includes an amplifier 14 configured toreceive the voltage signal provided by antenna 12. The output port ofamplifier 14 is coupled to a plurality of mixer circuits, such is 20 and22 respectively. An active control circuit 18 provides activationsignals via its output ports to mixer circuits 20 and 22. Each activatedmixer circuit receives a communication signal via antenna 12 andmultiplies that signal by a signal generated via a local oscillator (notshown).

The output port of each mixer circuit, such as 24 and 26 is coupled to acorresponding filter circuit such as 23 and 30. Each filter circuit isconfigured to provide a different prespecified bandpass frequency range.The output port of each filter circuit is coupled to an input port of alow frequency stage 40 of communication system 10, such as ports 32 and34.

Low frequency stage 40 includes a switch 36 configured to receive thesignal provided by one of the filter circuits such as 28 and 30. Theoutput port of switch 36 is coupled to an input port of an outputamplifier 38. The output port of amplifier 38 provides a filtered signaloutput that may be employed by the remainder circuitry of communicationsystem 10 (not shown).

During operation, active control circuit 18 activates one of the mixercircuits, such as 20 or 22, depending on the filter that is intended tobe used. The activated mixer circuit shifts the frequency range of thereceived signal via antenna 12 to a prespecified frequency range Theoutput port of the activated mixer circuit provides a voltage signal tothe input port of the filter that is intended to be used. For example,when active control circuit 18 activates mixer circuit 20, the outputport of mixer circuit 20 provides a voltage signal to filter 28 viaoutput port 24. Meanwhile, switch 36 is coupled to input port 32 so asto receive the filtered voltage signal provided by filter 28.

It is noted that each of the mixer circuits may be configured to shiftthe incoming communication signal via antenna 12, to the same frequencyrange or a different frequency range. This may be accomplished byemploying components in the mixer circuit with the same or differentcharacteristics. To this end, a first mixer circuit may provide a firstfrequency range. A second mixer circuit may provide a second frequencyrange that is substantially the same as the first frequency range or isdifferent from the first frequency range and so forth.

It is also noted that in accordance with another embodiment of theinvention, switch 36 may connect the output ports of filter 28 and 30 toamplifier 38 simultaneously. Because, only the activated mixer providesa voltage signal, the signal path comprising the non-activated mixer,and the corresponding filter would not carry a voltage signal.Therefore, amplifier 38 receives a voltage signal only from the paththat comprises an activated mixer and the corresponding coupled filter.However, this approach may cause cross-interference between permanentlyconnected filters, and may not be desirable in certain applications.

FIG. 2 is a schematic diagram of a mixer circuit referred to as aGilbert cell. The operation of Gilbert cells are well-known anddescribed in NEC product specification for Transistor Array UPA101B andUPA101G, and in NEC product specification for 900 MHZ Silicon MMIC DownConverter UPC1687G, and NEC product specification for Double-BalancedMixer and Oscillator NE612 (Nov. 3, 1987), all of which are incorporatedherein by reference.

As illustrated in FIG. 2, a Gilbert cell 58 includes at least twodifferential input ports such as 60-62 and 64-66, configured to receivethe signals which are intended to be multiplied. For example, ports60-62 are configured to receive a communication signal received viaantenna 12 of FIG. 1, and ports 64-66 are configured to receive amultiplying signal received from the output port of a local oscillator(not shown). Port 68 is configured to receive an activation signal so asto activate the multiplication operation of the Gilbert cell. Theresulting signal is provided at a differential output port such as70-72.

Differential input ports 60-62 provide the communication signal to acouple of differential pair amplifiers comprising of n-p-n bipolartransistors 80-82 and 84-86 respectively. The emitter terminals oftransistors 80 and 82 are coupled together. Similarly the emitterterminals of transistors 84 and 86 are coupled together. The baseterminal of transistor 80 and 86 are coupled together and to port 62.Similarly, base terminals of transfers 82 and 84 are coupled togetherand to port 60. The collector terminals of transistors 80 and 84 arecoupled together and to port 70. Similarly, the collector terminals oftransistors 82 and 86 are coupled together and to port 72.

A differential input pair comprising of transistors 78 and 76 areconfigured to receive multiplication signal via ports 64 and 66respectively. The emitter terminal of transistor 78 is coupled to theemitter terminal of transistor 76. The collector terminal of transistor78 is coupled to the emitter terminals of transistors 80 and 82.Similarly the collector terminal of transistor 76 is coupled to theemitter terminals of transistors 84 and 86. The base terminal oftransistor 78 is coupled to port 64. Similarly, the base terminal oftransistor 76 is coupled to port 66.

An activating switch defined by transistor 74 is configured to receivean activation signal via port 68 of Gilbert cell 58. The collectorterminal of transistor 74 is coupled to the common emitter terminal oftransistors 78 and 76 respectively. The collector terminal of transistor74 is coupled to ground.

During operation, all transistors are appropriately biased. A signalreceived at ports 60-62 is then multiplied by a signal received at ports64-66, whenever transistor 74 is “ON.” The resultant multiplied signalis provided at ports 70-72.

It is noted that the invention is not limited in scope to bipolartransistors employed in the exemplary Gilbert cell illustrated in FIG. 2and other types of transistors such as MOSFETs or FETs may be employedin accordance with other embodiments of the invention.

FIG. 3 illustrates a schematic diagram of a multiple Gilbert cell inaccordance with an exemplary embodiment of the present invention. Asillustrated in FIG. 3 a first Gilbert cell as described in FIG. 2 isemployed to receive signals at ports 60-62 and 64-66 respectively, andto provide a multiplied signal at output ports 70-72. A second Gilbertcell is coupled to the first Gilbert cell. Thus, a couple ofdifferential pairs comprising of transistors 110-112 and 114-116 areconfigured to receive a voltage signal via ports 60-62. The baseterminal of transistor 86 is coupled to the base terminal of transistor110 and transistor 116. Similarly, the base terminal of transistor 112is coupled to the base terminal of transistor 114 and to the baseterminals of transistors 84 and 82. The emitter terminals of transistors110 and 112 are coupled together, and, the emitter terminals oftransistors 114 and 116 are coupled together. The collector terminals oftransistors 110 and 114 are coupled to an output port 118. The collectorterminals of transistors 112 and 116 are coupled to an output port 120.

The base terminal of transistor 106 is coupled to the base terminal oftransistor 78 and to input port 64. Similarly, the base terminal oftransistor 108 is coupled to the base terminal of transistor 76 and toinput port 66. The collector terminal of transistor 106 is coupled tothe collector terminal of transistors 110 and 112. Similarly, thecollector terminal of transistor 108 is coupled to the emitter terminalof transistors 114 and 116.

The emitter terminal of transistors 106 and 108 are coupled together andto the collector terminal of an activating switch defined by transistor122. The base terminal of transistor 122 is coupled to receive anactivation signal via port 104.

During operation, each cell in the multiple Gilbert cell 102 may beactivated, while the rest are deactivated. For example, when transistor74 is “ON,” and transistor 122 is “OFF,” output ports 70-72 provide amultiplied signal resulting from the multiplication of signals receivedat ports 60-62 and 64-66. When transistor 122 is “ON,” and transistor 74is “OFF,” output ports 118-120 provide a multiplied signal resultingfrom the multiplication of signals received at ports 60-62 and 64-66respectively.

Output ports 70-72, in accordance with one embodiment of the presentinvention, may be coupled to a filter, such as 28 of FIG. 1. Similarly,output ports 118 and 120 may be coupled to a second filter, such as 30of FIG. 1, which has different frequency characteristics than filter 28.Whenever, it is desired to filter the incoming signal via filter 28, forexample, an activation signal is provided via port 68 so as to activatethe corresponding first Gilbert circuit, and a deactivation signal isprovided via port 104 so as to deactivate the corresponding secondGilbert circuit in multiple Gilbert circuit 102. Furthermore, wheneverit is desired to filter the incoming signal via filter 30, for example,an activation signal is provided via port 104 so as to activate thecorresponding second Gilbert circuit, and a deactivation signal isprovided via port 63 so as to deactivate the corresponding first Gilbertcircuit.

It is noted that the termination impedance of each of the Gilbert cellsemployed in a multiple Gilbert cell, such as 102 may be configured sothat it matches the input impedance of the corresponding filterconfigured to receive a signal from a Gilbert cell. In accordance withthis optimal matching arrangement, the distortion rate of signalstraveling through a Gilbert cell and its corresponding filter circuitreduces substantially.

It is further noted that although FIG. 3 illustrates a multiple Gilbertcell with two individual Gilbert cells, the invention is not limited inscope in that respect. For example, a variable bandwidth system may beemployed with a plurality of filters having different frequencycharacteristics. In that event, a multiple Gilbert cell having the samenumber of Gilbert cells as the number of filters may be employed. EachGilbert cell is coupled to a corresponding filter via its output ports.

FIG. 4 illustrates a schematic diagram of a multiple Gilbert cell 130employed in a communication system with variable filter bandwidth inaccordance with another embodiment of the present invention. Asillustrated the first and second mixer circuit in mixer cell 130 areactivated by an activation signal received via port 68.

The base terminals of transistors 78 and 106 are coupled to input port64 via biasing capacitors 146, and 138 respectively, and, the baseterminals of transistors 76 and 108 are coupled to input port 66 viabiasing capacitors 140 and 132 respectively. The base terminals oftransistors 78 and 76 are also coupled to activation port 68 via biasingresistors 144 and 142 respectively. The base terminals of transistors106 and 108 are coupled to activation port 68 via an inverter 124 andvia biasing resistors 136 and 144 respectively. Inverter 124 isconfigured to provide to the second mixer circuit an inverted version ofthe signal provided to the first mixer circuit. To this end, whenactivation signal provided to port 68 is “HIGH,” the first mixer circuitis activated and the second mixer circuit is deactivated. Conversely,when the activation signal provided to port 68 is “LOW,” the first mixercircuit is deactivated and the second mixer circuit is activated.

It is noted that in accordance with one embodiment of the presentinvention, the components described herein may be employed in anintegrated circuit arrangement.

Thus, in accordance with an exemplary embodiment of the presentinvention, a multiple Gilbert cell may be utilized to avoid the need ofa switch that couples the output of a mixer to one of the plurality offilters. The switching between the outputs of the multiple Gilbert cellis performed by activating one Gilbert cell and deactivating theplurality of the remaining Gilbert cells.

As mentioned before, instead of Gilbert cells, in accordance withanother embodiment of the invention, a plurality of active circuits suchas signal amplifiers may be configured to provide signals to acorresponding filter that receives the output signal of the activecircuit. The plurality of the active circuits receive the same inputsignal and only the active circuit that is turned “ON,” provides anoutput signal to the corresponding filter. The remaining active circuitsthat are turned “OFF,” provide no output signals to their correspondingfilters.

While only certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes orequivalents will now occur to those skilled in the art. It is therefore,to be understood that the appended claims are intended to cover all suchmodifications and changes as fall within the true spirit of theinvention.

1. A communication system with variable filter bandwidth comprises: afirst mixer circuit disposed within a high frequency integrated circuithaving input ports configured to receive a first communication signaland shift the frequency range of said communication signal to a firstfrequency range, wherein said first mixer circuit comprises a firstmixer; a second mixer circuit disposed within said high frequencyintegrated circuit having input ports configured to receive said firstcommunication signal and shift the frequency range of said firstcommunication signal to a second frequency range, wherein said secondmixer circuit comprises a second mixer; an amplifier coupled to saidfirst and second mixer circuits for providing said first communicationsignal to said first and second mixer circuits; an activation circuitcoupled to the first and second mixer circuits so as to provide anactivation signal that selectively activates any one of the mixercircuits; first and second filter circuits each configured to receive asignal from said first and second mixer circuits, when a correspondingone of said mixer circuits is activated and to provide a signal to a lowfrequency integrated circuit; and wherein when one of said mixercircuits is activated, the remaining mixer circuit does not generate anoutput voltage signal.
 2. The invention in accordance with claim 1wherein said first and second frequency range are substantially thesame.
 3. The invention in accordance with claim 1 wherein said filtercircuits are bandpass filters.
 4. The invention in accordance with claim3 wherein the frequency characteristics of said bandpass filters aredifferent from each other.
 5. The invention in accordance with claim 4,wherein the termination impedance of the output stage of each of saidmixer circuits substantially matches the termination impedance of theinput stage of each one of said bandpass filters.
 6. In a communicationsystem, a method for routing a signal provided by a mixer circuitdisposed in a high frequency integrated circuit to one of a plurality offilter circuits, said method comprising the steps of: receiving acommunication signal via a first amplifier and providing saidcommunication signal to a plurality of mixing circuits for shifting thefrequency range of said communication signal, wherein each of at leasttwo of said plurality of mixing circuits comprises a mixer; providing anactivation signal generated by an activation circuit that selectivelyactivates any one of said mixer circuits while one or more remainingmixer circuits do not generate an output voltage signal; and coupling aplurality of filter circuits to said mixer circuits such that each ofsaid filter circuits is configured to receive a signal from acorresponding mixer circuit, when said corresponding mixing circuit isactivated and to dispense a signal to a low frequency integratedcircuit.
 7. The method in accordance with claim 6 wherein said step ofshifting the frequency range further comprises the step of shifting thefrequency range via each mixer circuit to substantially the samefrequency range.
 8. The method in accordance with claim 7 furthercomprising the step of bandpass filtering said signal provided by saidactivated mixer circuit via a corresponding one of said filter circuits.9. The invention in accordance with claim 8, further comprising the stepof substantially matching the termination impedance of the output stageof each of said mixer circuits with the termination impedance of theinput stage of each one of said bandpass filters.